Design of products to be made from polymeric (plastic) materials requires consideration of the environment in which the product is to be used to guide selection of a polymer suitable for the end-use environment. A product designer gives consideration to properties such as optical clarity, chemical resistance, temperature resistance (both high and low), moisture uptake, modulus, etc. Preferably, the performance of the polymer in the environment of use can be ascertained to anticipate product performance and lifetime.
Properties that require consideration are polymer creep, stress relaxation, and, particularly for elastomeric articles, compression set and tensile set. Creep and stress relaxation are the main deformation mechanisms that polymers undergo. Creep of a polymer article occurs when a force is continuously applied to the article, causing it to deform gradually. The deformation of the polymer article is generally recoverable after the load is removed, but recovery takes place slowly with the polymer chains returning to their initial state. The rate at which polymers creep depends not only on the load, but also on temperature and, in general, a loaded component creeps faster at higher temperatures. Creep is readily measured using standard procedures, such as that set forth in ASTM D2990-01 or D790.
Stress relaxation is almost exclusively a characteristic of polymeric materials and is a consequence of delayed molecular motions as in creep. Contrary to creep, however, which is experienced when the load is constant, stress relaxation occurs when deformation (or strain) is constant and is manifested by a reduction in the force (stress) required to maintain a constant deformation. The degree to which stress relaxation is exhibited by a polymer depends on numerous factors such as polymer chemistry, molecular weight, type and amount of crystallinity in the polymer, temperature, environmental conditions, applied load and other factors. Often, it is not possible to readily predict what kind of stress relaxation a particular polymer material may exhibit until extensive testing and evaluation are conducted.
It is known that stress relaxation in polymers can be reduced by the addition of inorganic fillers, for example glass fibers, calcium carbonate or calcium silicate, carbon black, or other known reinforcing agents. However such reinforcing agents typically reduce ease of molding and compromise optical properties. For example addition of as little 5% of inorganic filler into a clear plastic typically gives rise to a highly opaque material. Stress relaxation can also be reduced by crosslinking a polymer, e.g. making a thermoset. Thermosets are generally more difficult to process than thermoplastics (U.S. Pat. No. 6,667,351).
Stress relaxation of molded or thermoformed parts can be differentiated from stress resulting from molding or machining. It is known that molding or machining polymers introduces stress, referred to as “molded-in” stress, where the article is, for example, more prone to cracking or crazing after machining. Such molded-in stresses can be removed by briefly heating the molded plastic parts (see, U.S. Pat. No. 5,324,473). Typically, the brief period of heating is conducted at a temperature that is above the glass transition temperature, but lower than the melt temperature or the decomposition temperature of the polymer. The presence or absence of internal stress in a polymer is readily ascertained by use of polarized light and is well known to those skilled in the art.
Elastomers are distinguished from other polymers by the criterion that most elastomers can be stretched at room temperature under a low stress and return to their original shapes after release of the stress. Many elastomers were developed either as cost competitive substitutes for vulcanized natural rubber or were formulated to meet a specific property requirement found lacking in natural rubber. Because most elastomers lend themselves to being compounded with many different materials, the choice of elastomer compounds available and the range of their uses is extensive. Products ranging from tires, to o-rings, to medical devices and children's toys, are formed from elastomers.
In designing articles from elastomers, compression set and tensile set are properties to consider. Compression set refers to the extent the elastomer is permanently deformed by a prolonged compressive load (ASTM D395). Tensile set refers to the extent the elastomer is permanently deformed after being stretched a specific amount for a short time, and is expressed as a percent of the original length or distance between gauge marks (ASTM D412). Failure of elastomeric articles exposed to a constant pressure or to repeated stretching and release, particularly when coupled with warm, humid environments, is a concern that must be considered when selecting a material for use in a given application.
U.S. Pat. No. 4,847,033 describes a process for decreasing the “free volume” of a polyethylene terephthalate (PET) film by first heating to a temperature below the glass transition (Tg), then tensioning the article, and then heating again below Tg. The heating process induces stress relaxation, i.e., induces a relaxation of the stresses in the polymer, and does not to prevent stress relaxation.
U.S. Pat. No. 4,900,502 discloses wet annealing below the glass transition temperature of microporous hollow fibers. The microporous fibers are formed of polysulfone, polyethersulfone, polyphenylsulfone, polyvinylidenefluoride, polyimide, or polyetherimide. The effects of heat treatment on microporous polymer articles does not relate to the effects of heat treatment on non-microporous polymer articles.
U.S. Pat. No. 6,171,758 describes annealing a semi-crystalline polymer by heating it to a temperature above the glass transition and below the melting point to improve dimensional stability.
U.S. Pat. No. 6,375,978 describes annealing of polymer membranes at temperatures 5-30° C. below the melting temperature of the polymer and above the glass transition temperature.
U.S. Pat. Nos. 4,816,342 and 4,770,944 describe wet annealing of ethylene vinyl alcohol co-polymers, polyvinyl alcohol, and nylon to increase crystallinity. The annealing is done at temperatures above the glass transition temperature.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.